Electroplating of chromium (Cr) is an important and mature technology that has been used in a variety of applications including metal and plastic plating and hard facing of finished parts. However, there has been a major concern over the waste stream that Cr plating generates. For example, contamination of ground water by Cr VI (hexavalent Cr) solutions was such a serious problem in Corvalis, Oreg., that a Superfund was created for clean up of that site. As a result of contamination problems of this type and the carcinogenic nature of some hexavalent Cr solutions, the cost of waste disposal has been steadily rising to the point where the overall cost of Cr plating has become prohibitive and uneconomical to many manufacturers. There exists a great need for a process for applying Cr to substrates without producing vast quantities of waste solution that must be properly disposed of at high cost.
Several techniques have been attempted to address this problem. For example, one technique involves processing of the electroplating bath and wash water through ion exchange columns to thereby chemically change Cr VI (hexavalent Cr) to Cr III (trivalent Cr), providing a recoverable by-product. Such a technique is disadvantageous in that the generated wash water solutions are too dilute and the amount of water used is so large as to make this technique industrially impractical. Such techniques are described by Leatherdale et al. in Society of Automotive Engineers Transactions, 100:1:1 (1991) #910920,1 and by Haung et al. in Journal of Chemical Engineering of Japan, 24:2 (1991) 149.
Another technique directed at solving the aforementioned problem involves changing the nature of the wet chemical bath so that Cr VI complex solutions are generated in smaller quantities than with use of earlier conventional Cr electroplating baths as described, for example, by Zaki et al. in Metal Finishing, 88:6 (1990) 99. In particular, trivalent Cr plating baths are described wherein precautions are taken to prevent or reduce oxidation of trivalent Cr to hexavalent Cr. While such a technique is inviting as a short term solution to the problem, it appears highly probable that chromium-bearing effluent solutions from such processes will be subject to strict environmental scrutiny in the future given the potential for toxity problems associated with heavy metal electroplating solutions.
An object of the present invention is to provide a deposition method, as well as coated substrate, that eliminates altogether the generation of heavy metal waste solutions associated heretofore with Cr electroplating and thereby eliminates the waste solution disposal problem.
Another object of the present invention is to provide a deposition method, as well as coated substrate, for Cr and alloys thereof and Cr-bearing compounds, such as oxides, nitrides and carbides, using chromium organometallic compounds thermally decomposed in an inductively coupled plasma.